Timing is everything: Drivers of interannual variability in blue whale migration

Angela R Szesciorka,Peter J Franks,Ana Širović,John Hildebrand,Ally Rice,Lisa T Ballance

doi:10.1121/1.5136714

Abstract

The growth and survival of large-bodied marine predators depend on temporal synchrony with resource availability. Baleen whales migrating long distances must therefore respond to interannual variability to avoid a predator-prey mismatch. Highly migratory and acoustically active, blue whales are a model species for investigating the drivers and timing of migration. Using passive acoustic recordings collected from 2007 to 2017, we examined the relationship among migration timing (inferred from blue whale “D” and “B” calls), environmental indices, and measured krill abundance. Arrival to Southern California feeding grounds was correlated with sea surface temperatures (SST) on Costa Rica breeding grounds (r = 0.81, <0.01). Colder SSTs in both regions resulted in early arrival and correlated with greater krill abundance in Southern California (r2 = 0.47, p = 0.03). The correlation between krill abundance and the transition from D to B calls (r2 = 0.55, <0.01) suggested that in high-krill-abundance years, whales switched earlier from social to reproductive-related behavior on their feeding grounds. This phenotypic plasticity may allow blue whales to accommodate environmental variability while balancing important biological needs. However, D call onset dates increased significantly over the 11-year study (r = 0.68, p = 0.03), shifting arrival two months earlier. Longer time on the feeding grounds may increase ship-strike and entanglement risk for this endangered species.

Highlights

Blue whales need to time their migration from their breeding grounds to their feeding grounds to avoid missing peak prey abundances, but the cues they use for this are unknown
We examine migration timing, environmental conditions, and prey during a 10 year period (2008–2017) in waters of the Southern California Region where blue whales feed in the summer
The 10-year average annual cycles of D and B calls through time indicates that whales arrive at the Southern California Region (SCR) feeding grounds in May and depart in November, remaining at the feeding grounds an average of 8.4 months (Fig. 1, Supplementary Table 1, Supplementary Fig. 3)

Summary

Introduction

Blue whales need to time their migration from their breeding grounds to their feeding grounds to avoid missing peak prey abundances, but the cues they use for this are unknown. Seasonal upwelling and the ensuing assemblage of zooplankton and forage fish create rich feeding grounds that are exploited by highly migratory predators[3,4] The timing of these physical-biological couplings is strongly influenced by environmental variability on interannual to multi-decadal scales[5]. Previous studies have found that the majority of these whales occupy feeding grounds in the CCE of the United States, including waters west of the Southern California Region (SCR) from May to December before migrating south to their breeding grounds in the Costa Rica Dome (CRD) for the winter where they reproduce or give birth (Supplementary Fig. 1)[14,15,17] The diet of this population in the SCR is overwhelmingly dominated by two species of euphausiid krill (Thysanoessa spinifera and Euphausia pacifica)[18,19,20]. We test the hypothesis that the timing and drivers of migration, including the transition from predominately feeding to reproductive-related behaviors, is mediated by available prey resources and physical environmental properties

Methods

Results

Conclusion